This invention relates to a switching device for operating a high-pressure discharge lamp with a voltage having successive periods of opposite polarity, comprising:
input terminals for connecting a power supply source,
output terminals for connecting the lamp to be operated,
a switch mode power supply and control means for controlling the switch mode power supply, and
means to form a lamp signal corresponding to the voltage across the lamp.
The invention also relates to a projection apparatus comprising a switching device which can suitably be used to operate a high-pressure discharge lamp.
A switching device of the type mentioned in the opening paragraph is known from WO 00/36882 (D98161) which corresponds to U.S. Pat. No. 6,232,725 which is hereby incorporated by reference into this application. The known switching device comprises means which enable the power supplied to the lamp to be controlled by means of a microprocessor in such a way that the shape of the current through the lamp can be adjusted for each period of the supply_voltage having successive periods of opposite polarity. The shape of the current flowing through the lamp is adjusted on the basis of the lamp signal corresponding to the voltage across the lamp. In this manner, flicker and unstable burning of the lamp can be substantially counteracted. The known switching device can particularly suitably be used to operate a high-pressure discharge lamp in a projection system, such as a projection television receiver.
A drawback of the known switching device, however, is that it is very complex and, in addition, offers no protection against failure of the lamp as a result of, for example, asymmetric operation of the lamp over a long period of time.
It is an object of the invention to provide a measure to counteract said drawback. This is achieved by means of a switching device of the type mentioned in the opening paragraph, which is characterized in accordance with the invention in that the switching device comprises means for detecting a Direct current through or a DC voltage across the high-pressure discharge lamp.
An advantage of the switching device in accordance with the invention resides in that undesirable, long-term asymmetry in the operation of the lamp can be detected in a comparatively simple way. Asymmetric operation of the lamp is based on the fact that the lamp effectively carries a DC voltage and/or a Direct current over a length of time covering at least two successive periods of opposite polarity. The detection of a Direct current through or a DC voltage across the high-pressure discharge lamp preferably takes place by means of the lamp signal. This has the advantage that the complexity of the switching device remains limited.
In an advantageous embodiment of the switching device in accordance with the invention, the formation of the lamp signal relates to time-sequential voltage detections, and the means for carrying out the detection include comparison means for comparing successive voltage detections. In this manner, a voltage difference between successive voltage detections can be advantageously established.
Preferably, the switching device is provided with a commutator (i.e. DC/AC converter) as in U.S. Pat. No. 6,232,725 that converts a DC current into an alternating current having successive periods of opposite polarity, and the lamp signal is formed between the output of the switch mode power supply and said commutator. This has the important advantage that improper functioning of the commutator is also detected. Improper functioning of the commutator, in general as a result of improper switching of one of the commutator switches, is an important cause of Direct current or DC voltage operation of the lamp. The possibility of monitoring the proper functioning of the commutator by means of thermal detection is impracticable, taking into consideration the fact that a commutator in a switching circuit for a high-pressure discharge lamp often comprises 4 switching elements, because this thermal detection would require at least 2, but preferably 4, thermal detection circuits, which would result in a very complex procedure.
In an advantageous embodiment of the switching device, the detection means can be used to carry out an algorithm wherein, after each voltage detection, a counting register is controlled on the basis of the outcome of a comparison between the established voltage difference and an average voltage and, upon the counting register exceeding a threshold value, a control signal is generated for switching the switch mode power supply to a stand-by mode.
Surprisingly, it has been found that as regards the successive voltage detections, a voltage detection in each period of the successive periods of opposite polarity is sufficient. As a result, the switching device remains comparatively simple. If the lamp signal to be detected contains comparatively much noise, then it may be advantageous to employ a filter circuit in the switching device, for example in the form of an analog low-pass filter. Another possible filter technique is a digital filter technique, for example for averaging out 2 or more voltage detections per period of the successive periods of opposite polarity.
Preferably, the average voltage is formed from a moving average of voltage detections in the last 4 successive periods of opposite polarity. This has the important advantage that the average voltage, which is used as a reference value, is related to each individual lamp. As a result, a spread in properties between individual lamps has no appreciable effect on the reliable functioning of the switching device.
In experiments it has been found that if the established voltage difference exceeds 50% of the average voltage, then this voltage difference can be used as a measure of the occurrence of a Direct current through or a DC voltage across the lamp. In such a case, an increase of the counting register by one enables the frequency of the direct current flowing through or the DC voltage applied across the lamp to be monitored. Preferably, the counting register is reduced by one if the established voltage difference is equal to or smaller than 50% of the average voltage. In this manner, the influence of accidental fluctuations in the current through or the voltage across the lamp is substantially eliminated in a simple, reliable manner.